Note: The above diagrams incorporate corrections made since the publication of the print version of this article in December CIBSE Journal. The print version contained some directional arrows that were the wrong way around. We apologise to the author for these printed errors.

temperature as the temperature will not be representative of the system load, due to the change in flow rate. Flow temperature control can be used, but is only normally stable where it is correctly integrated to control boilers with modulating burners. Flow temperature control of boilers with on/off or high/low burners can be unstable and can interact with the boiler thermostats, unless particular care is taken with operating temperatures, which must conform with Health and Safety Executive guidance note PM5 (www.hse.gov.uk). Boilers with individual pumps can be very effectively controlled with heat load control, which also allows the boilers to be controlled for their most efficient range of operation. Condensing boilers: Many condensing boiler installations rarely work in condensing mode as, apart from start- up, the return water temperature is often above 54C, due to the system design. Direct compensation of boilers can achieve significantly increased condensing operation, but can only be used where a quick response to increase temperature is possible for HWS demand, where required. System differential temperatures: Modern boilers are normally designed for higher

24 CIBSE Journal December 2011

differential temperatures, and consequently have lower flow rates. Whilst substitution of modern boilers with packaged pumps is normally successful at a domestic level, the effect on hydraulics must be considered for larger retrofit installations; reverse flows in common headers causing dilution of secondary flow temperatures is the most common problem. Boilers with variable flow pumps: Pumps that vary boiler flow in unison with burner firing to maintain constant differential temperatures can cause significant hydraulic and controllability issues in multiple boiler applications, particularly where system differential temperatures have also not been carefully considered. HWS segregation: Many systems combine heating and HWS heat sources to reduce capital costs. Segregation of HWS can often reduce standing losses in summer, and can permit far greater flexibility for boiler control, enabling more effective condensing operation. Two-stage burners: These are very rarely correctly commissioned; only a few months ago I saw new boilers with both high and low burner stages set to the same temperature, working as single stage burners. Where controlled by the individual boiler thermostat, the second stage must be set to a lower temperature than the first stage; consequently, significant flow temperature variations will occur. Control can be integrated with sequence controls on multiple boilers to work effectively, but care must be taken to ensure compliance with HSE guidance note PM5. There is little to justify two-stage

burners as the theoretical gains over single stage burners are very rarely achieved. Modulating burners should always be used for good control and energy efficient operation. Biomass boilers: These are increasingly used as a low carbon solution, but require a number of additional factors to be considered for successful operation. Purely adding a biomass boiler to an existing system without considering overall system integration – hydraulics and control – can be a recipe for problems. There are too many factors to consider here, but controllability is a major factor in successful biomass boiler system design. Component pressure loss: Modern boilers can have significant pressure losses, which should be considered in their selection, as this will add to overall energy consumption. System components should also be